Two events, unrelated though of similar scale, occurred in the complicated mix of splintered crust and microplates which characterises the boundary between the Pacific and Australian plates, north of Australia.
In the east of the region, an M6.0 tremor occurred on the edge of Latanga Island, Papua New Guinea while to the west an M6.1 struck at a depth of around 18km in the Molucca Sea.
No major damage was reported as a result of any of these earthquakes and, although the two larger occurred offshore, neither was of sufficient magnitude to generate a tsunami.
Earthquakes in Central America
Typically, the band along the western coast of Central America experiences regular earthquake as the eastwards-moving Cocos plate is forced against the stable North American and Caribbean plates. The result is that the denser Cocos plate is subducted and earthquakes occur along the boundary.
Previous weeks have seen a period of relative seismic calm in this region, but the week of 12-18 December saw 18 earthquakes of ≥M4.0 along the north-eastern boundary of the Cocos plate, affecting the countries of Mexico, Guatemala, El Salvador, Nicaragua and Costa Rica. The largest was an M5.5 off Guatemala, in whose territory or territorial waters seven of the tremors occurred. Five tremors, three of them of at least M5.0, occurred in or off Guatemala.
United States Earthquakes
Apart from California’s offshore M6.3 described above, the US (excluding Alaska) saw slightly elevated levels of activity across the country. The San Andreas Fault zone saw five tremors of between M3.0 and M3.7 between the Gulf of Mexico and the point at which it reaches the ocean to the south of Eureka. Elsewhere the generally stable tectonic continental interior saw minor earthquakes of M3.0 or more in Oklahoma, Montana and New Mexico.
United States Geological Survey Real time earthquake map. Accessed 18 December 2012
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